Neuromuscular disease, COPD and obese hyperventilation patients often suffer from chronic respiratory failure. Said patients need regular treatment of their respiratory failure at home. Hypoxemic patients are treated by oxygen therapy, mostly without ventilator support, while treatment by non-invasive ventilation with environmental air helps bringing the high carbon dioxide blood gas level of hypercapnic patients back to an acceptable level. The efficiency of the ventilation is checked by measuring the baseline and the trends in arterial oxygen and carbon dioxide levels during nocturnal NIV.
Transcutaneous CO2 sensors are used at home instead of arterial blood gas analysis. Arterial blood gas analysis is widely used, but needs sample taking by a professional, is painful and can have complications. Transcutaneous CO2 sensors are accepted for domiciliary care, and also in the hospital for CO2 monitoring.
To derive the transcutaneous CO2 value from the measured—cutaneous—partial CO2 pressure, the difference between the sensor temperature and the arterial blood temperature of 37° C. has to be taken into account. Furthermore, an offset is subtracted from the measured value to compensate for the skin metabolism that varies somewhat with skin temperature.
Arterialization of the skin is essential for transcutaneous blood gas measurements to obtain a transcutaneous value that reflects the arterial CO2 blood gas level. Existing technology is based on arterialization by heating the skin below the sensor surface. In currently available transcutaneous systems the minimal sensor temperature for stable arterialization is 42° C. and the required heating power is ˜500 mW at maximum, which is mainly needed to compensate for the cooling effect of the blood flow.
The prevention of skin-burns is a main concern in skin heating. As the heater is DC coupled, electronic failures in e.g. the driver circuitry or software errors may lead to uncontrollable heater currents. To safe-guard such situations a substantial amount of hardware is added to the main temperature control loop.
Chemo optical sensor materials may be applied to measure CO2 transcutaneously. Chemo optical sensor may comprise the following: On top of an optical transparent carrier material two layers of ‘silicon rubber-like’ gas-permeable materials are deposited. The first layer—the sensing layer—comprises a mixture of two fluorescent dyes in chemical buffer material; namely a reference dye having a long fluorescent life-time and a pH-sensitive indicator dye having a short life-time. The second layer—the membrane—comprises light reflecting material (TiO2) particles and prevents ion transport to and from the sensing layer. CO2 gas diffuses through said membrane into the first layer and changes the pH, which change the fluorescence from the indicator dye. By a dual life-time referencing technique, effectively measuring the time response to modulated light excitation, the percentage CO2 gas is calculated. At first sight the properties of these sensor spots look unmistakeable advantageous for the design of a disposable transcutaneous sensor device for the home market in terms of: dynamic range, pre-calibration, compensation for deviating temperature, stability, and cost-effectiveness.
Skin heating and contact fluid are essential to achieve sufficient and stable skin arterialization. For this purpose the sensor temperature must be accurately controlled in a range between 40˜45° C. In the ideal situation the foreseen nocturnal transcutaneous CO2 monitor system is wear-able, preferably cable-less and has a small form factor.